148 THE SUPERIOR EXTREMITY 



upon the volar surface of the radius; while the pronator 

 quadratus is a quadrate fleshy layer closely applied to both 

 bones immediately proximal to the wrist. The volar inter- 

 osseous artery and nerve proceed distally in the interval 

 between the flexor profundus and flexor pollicis longus. 



M. Flexor Digitorum Profundus. The deep flexor of 

 the fingers springs from the volar and medial surfaces of 

 the ulna in its proximal three -fourths. It derives fibres 

 also from the volar surface of the interosseous membrane 

 and the aponeurosis by which the flexor carpi ulnaris takes 

 origin from the dorsal border of the ulna. The fleshy mass 

 gives place to four tendons for the medial four digits, but 

 only one of them that for the index finger becomes 

 separate and distinct in the forearm. The tendons proceed 

 distally, deep to the transverse carpal ligament, into the palm. 

 The flexor digitorum profundus is supplied by the volar 

 interosseous branch of the median and by the ulnar nerve. It 

 is a flexor of all the joints of the fingers, and of the wrist. 



M. Flexor Pollicis Longus. The flexor pollicis longus 

 arises from the proximal two-thirds of the volar surface of 

 the radius. It takes origin also from the adjacent part of 

 the volar surface of the interosseous membrane. A rounded 

 tendon issues from the fleshy belly, and proceeds into the 

 palm, deep to the transverse carpal ligament. 



In many cases the flexor pollicis longus has an additional 

 slender head of origin, which springs from the medial border 

 of the coronoid process of the ulna, or the medial epicondyle 

 of the humerus. The flexor pollicis longus is supplied by 

 the volar interosseous nerve. It is a flexor of all the joints 

 of the thumb and of the wrist. 



M. Pronator Quadratus. The quadrate pronator muscle 

 takes origin from the distal fourth of the volar surface of 

 the ulna, and is inserted into the distal third of the volar 

 surface of the radius. It is supplied by the volar interosseous 

 nerve. It is pronator of the forearm and hand. 



Dissection. Divide the pronator quadratus vertically and 

 clean the part of the volar interosseous artery which lies 

 behind it. 



WRIST AND PALM. 

 In this dissection the following structures will be found : 



